Stockholder's equity (deficit) - Restricted stock activity narrative (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Unvested restricted common stock	155,149	80,190	226,400
Early exercise of unvested stock options	491,207
Restricted stock
Number of Shares
Unvested restricted common stock	155,149
Early exercise of unvested stock options	131,551
Restricted stock | Founders
Number of Shares
Unvested restricted common stock	23,598